Optical sensing technology as an important branch of information science technology has important applications in the fields of industry process control, environment monitoring, food security and homeland security. Optical sensing technology can solve problems of electrical sensing technology such as low sensitivity, interference, slow, insecurity for some chemical gases. Optical sensor has advantages of high sensitivity, compact, immunity to electromagnetic interference, suitable for integration and real time measurements, playing a more and more important role in the sensing field.
The basic principle of optical waveguide sensor is based on evanescent filed at the surface of fiber or planar waveguides. The interaction between evanescent filed and the analyte will influence the transmission characteristics of the optical waveguide, thereby achieving optical sensing by detecting the change of the transmitted light.
Ring resonator has attracted considerable attentions because it can provide a high sensitivity due to its sharp resonance peak. FIG. 1 shows the schematic diagram of an optical sensor based on a single ring resonator. The variation of the effective refractive index induces a shift in the transmission spectrum of the ring resonator. By measuring the peak wavelength shift or intensity change at a fixed wavelength around one transmission peak, the change of the analyte can be detected (K. De Vos et al, “Silicon-on-Insulator microring resonator for sensitive and label-free biosensing”, Optics Express 15, pp. 7610-7615, 2007). The drawback of the wavelength shift measurement is the need for an expensive optical spectrum analyzer (OSA), and the detection precision is directly related to the resolution of the OSA. In the case of measuring intensity at a fixed wavelength around a transmission peak, it needs a laser source with a narrow linewidth and an accurate wavelength relative to the position of the transmission peak, and a good stability. These requirements increase the cost of the measurement system, and decrease the reliability.